It sounds like you may have your paints too thin, to start with. Maybe not, but maybe so
For me, I use 4-5 drops paint, and 1 drop of flow improver/water (but that's for reaper paints - I find
gw paints to be really inconsistent consistency). I think flow improver is miracle stuff, and I feel like it improved my ability to paint without headaches 1000x.
YMMV, but it's worth trying if you can get a hold of some. Most paint lines sell small bottles of it for the same price as paint, or you can get it online or at an art store in "more than you'll ever need" amounts for not much more.
Paint, especially thinned, will not be 100% opaque unless you go for a lot of layers.
I'm not sure where the 'crust' might be coming from without a picture. I've never had anything like that happen to me :(